yes Michel is correct,sorry i didn't notice the co-worker credentials working fine.in this case you can do as Michel said,do sniffer capture when using the working and non working CCO'sthen you can spot the difference.if not maybe you can just check ...
if you are using proxy while manually connecting to the internet then you might want to configure that in the LMS settings.also just for checkup if there is a firewall inbetween then you might just have a look and see if its blocking something.
if there is no backup running and you need a new backup,-stop the LMS -delete the lock file-start the LMS-try the backup (make sure you have the needed space).